The present invention relates to a magnetic recording/reproducing apparatus and a magnetoresistive element, and more particularly to a high-density magnetic recording/reproducing apparatus and a method of manufacturing the same.
JP-A-9-16920 describes a spin-valve magnetoresistive sensor using a laminated antiparallel pinned layer and an antiferromagnetic exchange bias layer.
JP-A-7-169026 describes a spin-valve sensor using an antiferromagnetic coupling layer.
JP-A-6-236527 describes a magnetoresistive sensor having a conductive back layer at the back of a filter layer responsive to a magnetic field.
JP-A-6-111252 describes a magnetoresistive sensor having a soft-magnetic intermediate layer adhered between an antiferromagnetic layer and a ferromagnetic layer.
U.S. Pat. No. 5,768,071 describes a spin-valve type read head, in which exchange coupling is improved by inserting a thin, non-contiguous nonmagnetic layer of, for example, Cu between an antiferromagnetic film and a ferromagnetic film.
JP-A-2000-156530 describes a magnetoresistive element including a third layer of, e.g., oxide in a second magnetic layer with magnetization substantially fixed. Further, the Society of Japan Applied Magnetism, Summary Proceedings of 23rd Conference 6aA-5, describes a spin-valve film having a magnetization fixed layer containing a nano oxide layer.
Digests of Intermag 2000, FA-08, describes a giant magnetoresistance (GMR) film making use of a thin oxide. FA-07 of the same literature describes a GMR film having a protective oxide film laminated on a free layer. BQ-12 of the same literature also describes a GMR film having a protective oxide film laminated on a free layer. FA-09 of the same literature describes a spin-valve film using a magnetic oxide layer.
Digests of Intermag 1999, DB-01, describes a spin-valve film making use of a pinned layer having an oxide layer inserted therein.
Physical Review B53, 9108 (1996) and Journal of Applied Physics, 79, 5277 (1990) describe that the giant magnetoresistance is enhanced by an effect of an interface with an oxide.
In the prior art, it has been impossible to realize a magnetic recording apparatus with a sufficiently high recording density, in particular, magnetoresistive elements in a read unit of the apparatus, which act on the external magnetic field with adequate sensitivity and output, and further to obtain a favorable performance with good symmetry, so that it has been difficult to realize the function as a storage device.
In recent years, it has been known that a multi-layer film having ferromagnetic metal layers laminated with nonmagnetic metal layers therebetween is great in magnetoresistance, or a so-called giant magnetoresistance. In this case, the magnetoresistance is varied in electric resistance in accordance with an angle formed by magnetizations of the ferromagnetic layers separated from one another by the nonmagnetic layers. In the case of using the giant magnetoresistance for magnetoresistive elements, a structure called a spin valve has been proposed. More specifically, output can be obtained by providing a structure of antiferromagnetic film/ferromagnetic layer/nonmagnetic layer/soft-magnetic layer, using an exchange coupling magnetic field generated at the interface of antiferromagnetic film/ferromagnetic layer to substantially pin magnetization of the ferromagnetic layers in close contact with the antiferromagnetic films, and causing an external magnetic field to perform magnetizing rotation of the other soft-magnetic layer. The effect of the above pinning is called a pinning bias, and the antiferromagnetic film producing such effect is called a pinning bias film. Also, that ferromagnetic layer, for which the magnetization is substantially pinned, is called a pinned layer or a ferromagnetic pinned layer. Similarly, a soft-magnetic film caused by an external field to undergo rotation of magnetization is called a free layer or a soft-magnetic free layer. The pinned layer functions to be substantially pinned in magnetization relative to a magnetic field being sensed, and the antiferromagnetic film can be replaced by a hard magnetic film, that is, a material, for which magnetization will not change except for application of a relatively large magnetic field.
With a magnetic head using the spin-valve type magnetoresistive laminated film, important factors include a thickness of the pinned layer and an amount of its magnetization. That is, this is because with a magnetic head having opposite surfaces exposed, a magnetic field leaks from ends of pinned layers in accordance with an amount of magnetization of pinned layers, i.e., a product of magnetization and thickness as one of factors of symmetry in waveform of a read head relative to an external field to have an influence on a direction of magnetization of free layers. In keeping with an increased recording density of magnetic recording/reproducing apparatuses in recent years, spin-valve sensors serving as read elements have been increasingly made small in size and leakage of the magnetic field from ends of pinned layers has produced large influences. To cope with this, it is necessary to reduce an amount of magnetization of pinned layers, i.e., reduce pinned layers in thickness or magnetic flux density. Simply thinning of pinned layers in a spin-valve laminated film, however, magnifies the effect of surface scattering to cause reduction in MR ratio, which will in turn impair the performance of a read sensor. Similarly, when the pinned layers of a spin-valve laminated film are modified in composition so as to be reduced in magnetic flux density, the giant magnetoresistance is impaired to cause reduction in MR ratio, which will also impair the performance of a read sensor. These problems are also observed with the free layer. JP-A-6-236527 proposes a construction, in which a conductive layer is provided behind a free layer called a filter layer to suppress reduction in MR ratio due to the surface scattering effect. Also, Physical Review and Journal of Applied Physics have pointed out that the effect of oxide causes an improvement in giant magnetoresistance. Further, the summary proceedings of conference of the Society of Japan Applied Magnetism has reported that a spin-valve film with oxide layers formed in pinned layers is high in MR ratio.
When an oxide and an antiparallel coupling layer are inserted in a pinned layer so as to apply a spin-valve type magnetoresistive laminated film to a magnetic head, in particular to a magnetic head for magnetic recording/reproducing apparatuses of high recording density, however, it is easily expected that a pinned layer will be of complex construction consisting of at least three of the magnetic films and oxide films and further five or more layers consisting of antiparallel coupling layers laminated. Such complex structure of a pinned layer can be an obstacle to attaining the function of a magnetoresistive element and makes it difficult to induce and produce a pinning bias essential for the operation of a spin-valve structure. For the magnetic head to function, a pinning bias is applied in a predetermined direction as described above. That is, a pinning bias is applied in a predetermined direction described above upon functioning as a magnetic head. A method of manufacturing a magnetic head comprises applying a magnetic field to a magnetoresistive film and performing heat treatment in a state, in which magnetization of a magnetic film in contact with an antiferromagnetic film is saturated in a predetermined direction described above, to apply a pinning bias. However, since the pinned layer is of complex structure and contains therein an antiparallel coupling layer having the property of firmly maintaining the magnetized state of an adjacent magnetic film in an antiparallel direction, a necessary performance cannot be obtained even when a magnetoresistive element is formed only by combining conventional techniques.
It is therefore an object of the present invention to provide a magnetic recording apparatus or a magnetic head using a magnetic sensor with a long-term reliability conformed to high-density recording. More specifically, an object of the present invention is to provide a spin-valve type magnetic sensor, of which amount of magnetization is reduced by inserting a thin oxide layer and an antiparallel coupling layer into a pinned layer using a bias application means such as an antiferromagnetic film partly and which is high in MR ratio, a construction, in which a pinning bias is correctly applied in a predetermined direction, a method of manufacturing the same, and a magnetic recording/reproducing apparatus, in which the construction is applied in a magnetic head.
To provide a magnetic recording apparatus, in which a magnetic sensor using a giant magnetoresistance conformed to high-density recording is mounted on a magnetic head, the present invention uses as the magnetic sensor a spin-valve type giant magnetoresistive laminated film, i.e., a magnetoresistive element of multilayer structure comprising an antiferromagnetic film/a ferromagnetic pinned layer/a nonmagnetic conductive layer/a soft-magnetic free layer. Here, the antiferromagnetic film serves to apply an exchange coupling bias for substantially pining the magnetization of the ferromagnetic pinned layer and may either be formed in direct contact with the ferromagnetic pinned layer or produce the similar effect indirectly through a magnetic coupling. Instead of the antiferromagnetic film, other bias application means such as the remanent magnetization of a hard magnetic film and a current bias may be used. To solve the above problems and to provide a magnetic recording/reproducing apparatus mounting thereon a magnetic head and a magnetic sensor which are conformed to high recording density, the present invention employs a structure in which the ferromagnetic pinned layer is formed as a laminate consisting of a first ferromagnetic film/an antiparallel coupling layer/a second ferromagnetic film/an oxide layer/a third ferromagnetic film. Here, the first ferromagnetic film making up the pinned layer contacts with the antiferromagnetic film and the third ferromagnetic film contacts with the nonmagnetic conductive layer. The antiparallel coupling layer is formed from a material of a thickness which antiferromagnetically couple the adjoining ferromagnetic films so that their magnetizations are oriented antiparallel to one another. Also, the oxide layer is formed from a material of a thickness which the adjoining second and third ferromagnetic films ferromagnetically couple so that their magnetizations are oriented parallel to a magnetic field to be sensed (i.e., the field from the magnetic recording medium). The oxide layer further forms smooth interfaces between them and the adjoining ferromagnetic films and functions to amplify a magnetic scattering phenomenon generated near the nonmagnetic conductive layer. To provide for magnetization of the ferromagnetic pinned layer of this multilayer structure in a predetermined state, an appropriate magnetic field is applied to saturate the first and second ferromagnetic films. The appropriate magnetic field means a field determined by the exchange coupling energy generated by the antiparallel coupling layer, the amount of magnetization of the first ferromagnetic film and the amount of magnetization of the second ferromagnetic film. This appropriate magnetic field functions to orient in a predetermined direction the magnetization direction of the first ferromagnetic film that should be substantially pinned by the exchange coupling field of the antiferromagnetic film. In this magnetic field, heat treatment for conducting the exchange coupling field from the antiferromagnetic film in a predetermined direction is carried out in a vacuum or in an inert gas.
In the above structure, there are two ways to conduct the magnetization direction of the first ferromagnetic film in a predetermined direction. One is to apply a sufficiently large magnetic field to saturate the first and second ferromagnetic films. In this case, the necessary magnetic field is represented as follows.
Hs=xe2x88x92xcexc0xc2x7Jxc3x97(M1xc2x7t1+M2xc2x7t2)/(M1xc2x7M2xc2x7t1xc2x7t2)
When the first and second ferromagnetic films have the same saturation flux density M,
Hs=xe2x88x92xcexc0xc2x7Jxc3x97(t1+t2)/(t1xc2x7t2)/M
Here, Hs is a saturation field, J is an exchange coupling energy induced by the antiparallel coupling layer, M1 and M2 are saturation flux densities of the first and second ferromagnetic films, and t1 and t2 are thicknesses of the first and second ferromagnetic films. Typical structure and values are as follows. A Ru antiparallel coupling layer has a thickness of 0.8 nm; and when the first and second ferromagnetic films are made of Co alloy, J=xe2x88x921.0xc3x9710xe2x88x923 J/m2, and M1=M2=M=1.8 T. Hence, when t1=2xc3x9710xe2x88x929 m and t2=1xc3x9710xe2x88x929 m, then Hs is about 1.0 T. For simplicity, the magnetic field is represented in a spatial flux density and in unit of tesla. It is seen from the above that it is necessary to apply a field of 1.0 T or higher for the magnetizing process of the magnetoresistive laminated film of the structure described above. Similarly, when t1=1xc3x9710xe2x88x929 m and t2=0.5xc3x9710xe2x88x929 m, Hs reaches as high as 1.8 T, which requires a magnetization heat treatment mechanism for generating a larger field than normally used, e.g., an in-field heat treatment system using a superconductive magnet. Further, in the magnetic head, the magnetoresistive laminated film is generally installed in a gap between a pair of soft-magnetic films called magnetic shields, so a substantial external field for effectively applying a field to the magnetoresistive laminated film should be in some cases considered to be substantially 1.5 times larger.
Subsequently, an explanation will be given to a way to apply a relatively weak field as a second way to conduct the magnetization directions of the ferromagnetic films in a predetermined direction. When a weak field of about several tens of mT is applied, the magnetization directions of the first and second ferromagnetic films turn while kept in their antiparallel state relative to each other, to be aligned so that a difference between the magnetizations is directed in the direction of the field. Therefore, the magnetization direction of the first ferromagnetic film can be specified for magnetization heat treatment by setting a film thickness difference between the first and second ferromagnetic films so as to produce a difference between amounts of their magnetizations, and applying a magnetic field which is sufficiently small relative to the antiferromagnetic coupling field of the antiparallel coupling layer and large enough to cause a magnetization difference between amounts of magnetizations of the first and second ferromagnetic films to align in the direction of field. A specific, appropriate magnitude of the field is of course determined by the saturation flux density of the magnetic films, settings of film thickness and the magnitude of the exchange coupling energy of the antiparallel coupling layer, but may be preferably in the range from about 0.01 T to 0.1 T.
Also, according to this invention, as another means for solving the problem described above, the ferromagnetic pinned layer is formed to be a laminate consisting of a first ferromagnetic film/a magnetization field control layer/a second ferromagnetic film/an antiparallel coupling layer/a third ferromagnetic film. The first ferromagnetic film making up the ferromagnetic pinned layer contact with an antiferromagnetic film, and the third ferromagnetic film contacts with a nonmagnetic conductive layer. As another means, the ferromagnetic pinned layer is formed to be a laminate consisting of a first ferromagnetic film/a magnetization field control layer/a second ferromagnetic film/an antiparallel coupling layer/a third ferromagnetic film/an oxide layer/a fourth ferromagnetic film. The first ferromagnetic film making up the ferromagnetic pinned layer contacts with an antiferromagnetic film, and the fourth ferromagnetic film contacts with a nonmagnetic conductive layer. The antiparallel coupling layer is formed from a material of a thickness such that the adjoining ferromagnetic films antiferromagnetically couple with the antiparallel coupling layer therebetween so as to have their magnetizations oriented antiparallel to each other. Also, the oxide layer is formed from a material of a thickness such that the adjoining ferromagnetic films ferromagnetically couple so as to have their magnetizations oriented parallel to a magnetic field to be sensed. The oxide layer further forms smooth interfaces between it and the adjoining ferromagnetic films and functions to amplify a magnetic scattering phenomenon generated near the nonmagnetic conductive layer. Further, the magnetization field control layer generates a ferromagnetic coupling in such a manner to have the magnetizations of the adjoining ferromagnetic films oriented parallel to each other. The magnitude of this ferromagnetic coupling is set relatively weak as compared with the coupling of the antiparallel coupling layer. Thereby, the magnetization of the first ferromagnetic film formed between the magnetization field control layer and the antiferromagnetic film can be saturated with a relatively weak field, i.e., a field sufficiently weak as compared with that required to saturate the magnetization of the ferromagnetic film adjoining the antiparallel coupling layer. In this case, the field required for the magnetization heat treatment is about 0.1 T to 1 T.
By determining various structures and magnetization means as described above, it is possible to realize a structure, in which the ferromagnetic film/oxide layer interface are formed in the ferromagnetic pinned layer to enhance the electron spin-dependent scattering effect, the magnetic film making up the pinned layer is properly prescribed in a substantial amount of magnetization and the exchange coupling field is applied to the magnetic films in the adjoining pinned layers of the antiferromagnetic film in a predetermined direction without dispersion to effectively pick up a magnetic field to be sensed, as a resistance change.
To amplify the giant magnetoresistance, materials similar to a ferromagnetic material forming the pinned layer and free layer, i.e., oxides of Co, Fe and Ni or a mixture of these oxides are suitable as materials of the oxide layer. Fe3O4 (magnetite) and CoFeO are particularly suited. While the oxide layer may be formed by oxidizing a magnetic film, the composition, thickness and structure of the oxide layer can be well controlled when it is formed by sputtering using an oxide target.
Ru, Os, Ir, Re and Rh, and alloys of these are suitable as materials of the antiparallel coupling layer. Further, other appropriate elements may advantageously be added for adjusting the exchange coupling energy of antiparallel coupling. Preferably, Pt, Cu, Au, Ag, Pd, Ni, Co and Fe are added in about 1 atomic % or more and 50 atomic % or less.
Non-ferromagnetic metals such as Cu, Ru, Pd and Cr, alloys thereof and oxides of 3d transition elements are preferable as materials of the magnetization field control layer. In particular, the use of oxides is advantageous as shunt currents are reduced. It is also preferable to use oxides having a crystallinity improvement effect and an electron reflection effect, which intensify the giant magnetoresistance.
In this invention, the ferromagnetic pinned layer and the soft-magnetic free layer are desirably made from alloys of Ni, Fe and Co or may be a laminated film of these alloys. The soft-magnetic free layer is desirably composed of a laminated film of Ni80Fe20 (3 nm)/Co90Fe10 (0.5 nm) with a CoFe arranged on the side of the nonmagnetic intermediate layer to gain a soft magnetism characteristic and increase in the giant magnetoresistance. Alternatively, a Co85Fe15 alloy thin film having a thickness of about 2 nm may be used for the soft-magnetic free layer, and further a Co alloy thin film with an appropriate oxide film inserted therein may be used therefor.
According to the present invention, it is possible to realize a high-density recording, i.e., one having a short recording wavelength recorded in a recording medium and recording tracks of narrow width for a sufficient read output and favorable recording with a magnetoresistive sensor using these materials and structures and a magnetic recording/reproducing apparatus whose read part is manufactured by subjecting the sensor to an in-field heat treatment that induces anisotropy in a predetermined direction.